1. Field of the Invention
The present invention relates to scanning probe microscopes (SPMs) including atomic force microscopes (AFMs) and, particularly, to an AFM that has a hermetically sealed superstructure that allows analyzation of a sample under different environmental conditions.
2. Description of the Related Art
An Atomic Force Microscope (“AFM”), as described for example, in U.S. Pat. No. RE34,489 to Hansma et al. (“Hansma”), is a type of scanning probe microscope (“SPM”). AFMs are high-resolution surface measuring instruments. Two general types of AFMs include contact mode (also known as repulsive mode) AFMs, and cyclical mode AFMs (periodically referred to herein as TappingMode.TM. AFMs). (Note that TappingMode.TM. is a registered trademark of the present assignee.)
The contact mode AFM is described in detail in Hansma. Generally, the contact mode AFM is characterized by a probe having a bendable cantilever and a tip. The AFM operates by placing the tip directly on a sample surface and then scanning the surface laterally. When scanning, the cantilever bends in response to sample surface height variations, which are then monitored by an AFM deflection detection system to map the sample surface. The deflection detection system of such contact mode AFMs is typically an optical beam system, as described in Hansma.
Typically, the height of the fixed end of the cantilever relative to the sample surface is adjusted with feedback signals that operate to maintain a predetermined amount of cantilever bending during lateral scanning. This predetermined amount of cantilever bending has a desired value, called the set-point. Typically, a reference signal for producing the set-point amount of cantilever bending is applied to one input of a feedback loop. By applying the feedback signals generated by the feedback loop to an actuator within the system, and therefore adjusting the relative height between the cantilever and the sample, cantilever deflection can be kept constant at the set-point value. By plotting the adjustment amount (as obtained by monitoring the feedback signals applied to maintain cantilever bending at the set-point value) versus lateral position of the cantilever tip, a map of the sample surface can be created.
The second general category of AFMs, i.e., cyclical mode or TappingMode.TM. AFMs, utilize oscillation of a cantilever to, among other things, reduce the forces exerted on a sample during scanning so as to minimize tip and/or sample damage, for example. In contrast to contact mode AFMs, the probe tip in cyclical mode makes contact with the sample surface or otherwise interacts with it only intermittently as the tip is scanned across the surface. Cyclical mode AFMs are described in U.S. Pat. Nos. Re 36,488, 5,226,801, 5,412,980 and 5,415,027 to Elings et al.
In U.S. Pat. No. 5,412,980, a cyclical mode AFM is disclosed in which a probe is oscillated at or near a resonant frequency of the cantilever. When imaging in cyclical mode, there is a desired tip oscillation amplitude associated with the particular cantilever used, similar to the desired amount of cantilever deflection in contact mode. This desired amplitude of cantilever oscillation is typically kept constant at a desired set-point value. In operation, this is accomplished through the use of a feedback loop having a set-point input for receiving a signal corresponding to the desired amplitude of oscillation. The feedback circuit adjusts the vertical position of either the cantilever mount or the sample by applying a feedback control signal to a Z axis actuator so as to cause the probe to follow the topography of the sample surface.
Typically, the tip's oscillation amplitude is set to be greater than 20 nm peak-to-peak to maintain the energy in the cantilever arm at a much higher value than the energy that the cantilever loses in each cycle by striking or otherwise interacting with the sample surface. This provides the added benefit of preventing the probe tip from sticking to the sample surface. Ultimately, to obtain sample height data, cyclical mode AFMs monitor the Z actuator feedback control signal that is produced to maintain the established set-point. A detected change in the oscillation amplitude of the tip and the resulting feedback control signal are indicative of a particular surface topography characteristic. By plotting these changes versus the lateral position of the cantilever, a map of the surface of the sample can be generated.
Notably, AFMs have become accepted as a useful metrology tool in manufacturing environments in the integrated circuit and data storage industries. A limiting factor to the more extensive use of the AFM was the inability to change the environmental conditions in which a particular sample positioned within the AFM is analyzed. For example, due to various operating conditions, it is desirable to determine the effects that these conditions will have upon various samples, such as elevated or reduced temperatures and pressures. However, because most AFMs are constructed to be utilized only at ambient temperatures and pressures, such AFMs are not sealed as environmental conditions experienced by the sample do need to be altered from the ambient. Although attempts have been made to offer environmental capability, there are significant drawbacks associated with performing tests under varying environmental conditions with existing AFMs.
More particularly, some recent AFM designs have been adapted to enable samples to be tested under varying environmental conditions. Examples of AFMs having this capability are described herein. In one known system, a cover is releasably and sealingly engaged with a chamber containing the sample to be scanned. The cover also includes resilient seals disposed around the moving parts of the AFM that extend into the chamber, such as the screws or Z actuator(s) and the cantilever tube. The sealing engagement of the cover with the moving parts and the chamber enables the sample contained within the chamber to be placed within a number of different environmental conditions, such as under a protective fluid, in the presence of a particular gas, or the like.
However, due to the imperfect seal created between the cover and the chamber, it is difficult to obtain stable variations in the environment surrounding the sample. For example, any gases discharged to the chamber may slowly escape overtime from the AFM past the seals created between the chamber and the cover if the seal is not formed correctly when the cover is engaged with the chamber, or when the seals surrounding the moving parts are disturbed when those parts are moving. Further, because the seal between the cover and the instruments and chamber is not hermetic, the seal can only withstand a certain pressure differential before failure, such that very low pressure environmental conditions, such as a vacuum, cannot be established within the AFM environment.
In another system, all of the components of the AFM are contained within a sealed chamber such as a bell jar. The AFM components are monitored by a controller connected to the components by feed throughs extending through the base of the bell jar in order to operate the AFM as needed. Further, gas inlets and outlets can be extended through the bell jar base to enable the environmental conditions within the bell jar to be altered, such as by introducing a specific gas, or removing all gases present within the jar to form a vacuum.
In another similar AFM, each of the components of the AFM are disposed within a bell jar that is sealingly connected to a base to completely enclose the interior of the jar. The AFM components are controlled using feed throughs extending through the base and connected to an exterior controller in order to conduct the analysis of the sample located inside the bell jar. Using the feed throughs, various environmental conditions can be created within the bell jar such that a sample can be evaluated in each of the different conditions.
Because each of the above-mentioned AFM is constructed using a sealed bell jar, they are capable of changing the environmental conditions present within the AFM. However, the construction of these types of AFMs makes it highly difficult to either change or alter the position of a sample being analyzed within the AFM that is undergoing analysis, such as to measure different selected areas of the sample surface or to compensate for sample drift. This is because the construction of the prior art bell jar AFMs requires that the seal between the cover and the bell jar be broken in order to access the sample disposed within the AFM. In doing so, the environmental conditions formed within the AFM are necessarily dissipated, such that once the sample is either changed or repositioned, the AFM must be resealed and the desired environmental conditions must be regenerated within the AFM. Thus, much time and effort is exhausted in simply duplicating the environmental conditions within the AFM.
Also, with regard to the positioning of the probe or cantilever tip with respect to the sample, most prior art AFMs utilize a piezoelectric element in order to move the cantilever closer to or away from the sample. Piezoelectric elements are normally used for this purpose due to the fact that the elements can be actuated by the application of a voltage to the element in order to move the cantilever very small distances, on the order of around one micron, in order to engage the sample as needed.
However, based on the inherent construction of the piezoelectric elements used, the elements are subject to a certain amount of error with regard to the distance that the cantilever is moved by the element. For example, if an element is sent a specific voltage differential in order to actuate the element and move the cantilever a specified distance, the actuation of the element may be accompanied by noise which causes the element to move the cantilever a distance slightly greater or less than that specified. When the distance the cantilever is to be moved is significantly greater than the error factor, the error factor does not significantly effect the position of the cantilever. However, when the cantilever is only to be moved a very short distance, the error factor can greatly effect the positioning of the cantilever. This can cause inaccurate or even worthless data. This problem is of particular concern when the environmental conditions associated with the experiment are altered from the ambient; for example, thermal drift can exacerbate positioning errors.
In sum, one significant drawback of prior art AFMs is that the sample cannot be readily manipulated within the AFM when a non-ambient environmental condition, such as reduced pressure or temperature is present within the AFM. As a result, the metrology field was in need of an AFM having the capability of altering the position of the sample within the AFM without interrupting the environment created within the AFM.
Another drawback of prior art AFMs is that the piezoelectric elements utilized to move the cantilever with respect to the sample when scanning the sample are greatly affected by noise in the voltage differentials applied to the elements to move the cantilever only a small distance. Therefore, it is also desirable to provide an improved piezoelectric actuator which greatly reduces the effect of the noise of the system on the movement of the cantilever by the actuator over a short distance.